Concerning (REF ), various closure approaches have been developed, including the Recent Fluid Deformation Approximation (RFDA) [1]}, Random Gaussian Fields Closure (RGFC) [2]}, and the Recent Deformation of Gaussian Fields (RDGF) closure [3]}. More recently, a multi-level version of the RDGF model has emerged (referred to as ML-RDGF) that provides a model for \(D_t{A}\)  that is valid for arbitrary Reynolds numbers. These closures for (REF ) all lead to a model for \({A}\)  that can generate steady-state statistics. This is in contrast to the Restricted Euler (RE) model [4]}, [5]} that ignores the anisotropic contribution to \({H}\)  and sets \(\nu =0\) , leading to a model for \({A}\)  that exhibits a finite-time singularity. Subsequent studies found that the addition of viscous effects to the RE model is not sufficient to prevent a finite-time singularity; the anisotropic pressure Hessian must also be accounted for. This could point to a potential challenge in closing (REF ); if the closure approximation for \(\kappa \nabla ^2{B}\)  is not sufficiently accurate then the predictions from the resulting model could also generate finite-time singular solutions for \({B}\)  since the equation for \({B}\)  contains no pressure Hessian to regulate the amplification term \({B\cdot A}\)  that causes \(\Vert {B}\Vert \)  to grow when \({B\cdot }({B\cdot A})<0\) . In this sense, developing a suitable closure for (REF ) may be more challenging than that for (REF ).
